The following sites define "overselling":
http://webhostinggeeks.com/articles/web-hosting/30810.phphttp://hostingfu.com/article/any-non-overselling-web-hosting-out-therehttp://hostingfu.com/article/overselling-according-to-whthttp://www.oversellingtalk.comOverselling is a term used in the web hosting industry to describe a situation in which a company provides hosting plans that are unsustainable if every one of its customers uses the full extent of services advertised. The term is usually referred to the web space and bandwidth transfer allowance.
-
WikiPedia